Ultrasonic vibrations for testing a structure have been used for many years in the field of Non-Destructive Testing (NDT).
Available ultrasonic vibration probes usually operate in the ultrasonic frequency range of 100 kHz to 10 MHz or higher. However, some materials do not respond well to such high frequency excitations and the use of lower frequency vibrations is advantageous. Application areas for low frequency NDT include advanced lightweight laminates and composite sandwich panels typically found in aerospace and marine structures.
The most popular low frequency technique is the so-called “Pitch-Catch” technique, where a probe comprises two identical transducers with one transducer set as a dedicated transmitter (pitcher) and the another transducer as a dedicated receiver (catcher). Such a “Pitch-Catch” NDT system can detect the mechanical response of a test structure and comparing this response to a response of a known reference item provides information concerning possible damage of the test structure.
A Pitch-Catch transducer may include a circular Lead-Zirconate-Titanate (PZT) bending actuator held either at its edges, or sandwiched between a compliant material such as a pair of rubber O-rings. The PZT elements are coupled to a test structure through a thick pin held in place with O-rings, or a rubber or cork sleeve so that the pin may be replaced when worn. Although manufacturers of these transducers are often quoting a usable frequency range of up to 70 kHz, inherent resonant artefacts within the designs are usually present and can interfere with measurements within the stated frequency range. Further, the designs of these transducers are often asymmetrical with electrical contact wires soldered to the disc. Asymmetry can contribute to degrading of the harmonic response of the transducer.
A low frequency NDT probe should ideally be used in a region of linear response away from any resonant artefacts of the probe. However, resonant artefacts between 20 and 30 kHz of known Pitch-Catch probes limit their linear frequency range to approximately 15 kHz. For many applications this limitation is of disadvantage and consequently there is a need for technological advancement.